In the manufacture of, for example, a fluorescent lamp, a gas such as argon is sealed in a tubular bulb at a prescribed pressure. One of the known gas-sealing processes is the co-called flow evacuation method which comprises introducing the same kind of gas as is to be sealed into one end of a tubular bulb in the capacity of a purge gas, thereby expelling impure gases such as air and nitrogen from the tubular bulb. After replacing the impure gases by the desired gas, an exhaust tube is sealed. With this flow evacuation method, the pressure of the purge gas in a tubular bulb during flow evacuation gives rise to variations of, for example, 5.+-.2 torr depending on the different diameters of an exhaust tube. If, in this case, the purge gas itself is introduced as a seal gas, then the pressure of the seal gas is subject to wide variations.
To avoid such an undesirable event, the conventional process comprises reducing the gas pressure in the tubular bulb to a level of about 0.5 torr after flow evacuation, introducing a desired gas into the bulb at a pressure of, for example, about 2.5 torr and thereafter sealing the exhaust tube. In this case, however, an appreciable length of time is consumed to decompress the interior of the tubular bulb (it takes about 30 seconds to reduce the gas pressure in the tubular bulb from, for example, 5 torr to a level of 0.5 torr), thus decreasing the operation efficiency of high output production. Further, this method, which comprises decompressing the interior of a tubular bulb and again sealing a gas therein, is undesirably complicated.
It is accordingly the object of this invention to provide a method and apparatus for easily and quickly stabilizing the pressure of a gas to be sealed in a tubular bulb to a prescribed level.